The invention relates to a method of cooling blown film comprising thermoplastic plastics during the production of blown film by means of a blown film extruder with a film blowing head which comprises an annular nozzle from which the blown film emerges and is guided away in an extraction direction. At least two cooling gas flows are made to approach the blown film from the outside in at least two blowing-out planes located at a distance from the annular nozzle. The invention also relates to a device for cooling blown film comprising of thermoplastic plastics during the production of blown film arranged at a blown film extruder with a film blowing head comprising an annular nozzle from which the blown film emerges and is guided away in an extraction direction. At least two annular cooling gas nozzles are arranged at a distance from the annular nozzle from at least two blowing-out planes for cooling gas.
The blown film can be produced and extracted along a vertical longitudinal axis A, and, an extraction direction can extend from the bottom to the top. The annular nozzle can be aligned in a corresponding direction. Alternatively, if the annular nozzle is suitably aligned, the extraction direction can be orientated from the top to the bottom.
As described herein, the extraction direction is generally provided from the bottom to the top. However, alternative embodiments and methods according to the invention can be provided wherein the extraction direction is from the top to the bottom.
For the output performance and thus for the economic efficiency of the above-mentioned method of producing blown film and of the devices involved, the process of cooling the blown film at the time of its exit from the annular nozzle up to the point in time of reaching a freezing limit comprises an important factor, wherein output rate can be increased by improving the cooling effect.
For cooling purposes, dual cooling rings can be provided and which can be arranged directly above the annular nozzle and can comprise a uniform cooling ring housing. Cooling gas nozzles can be provided, which form two blowing-out planes for cooling gas. An example of cooling gas is described in U.S. Pat. No. 5,804,221 A.
For improving the cooling effect further, double cooling rings can be provided wherein two independent cooling ring housings can be arranged one behind the other and can be spaced from one another in the direction of production, with the lower cooling ring being mounted directly on the film blowing head and with the upper cooling ring being arranged in such a way that its height is adjustable relative to the lower cooling ring. When the blown film enters the upper cooling ring, it can have been pre-cooled by the lower cooling ring. In this case, the effect of the upper cooling ring can be adversely affected in that the cooling gas emerging from the lower cooling ring has already been heated along the blown film and enters the upper cooling ring from below. Furthermore, access to the annular nozzle, which is important when starting the system, can be complicated by the lower cooling ring positioned on the film blowing head. A similar such device is described in EP 1 719 602 A1 for example.
In DE 32 43 884 A1, a method is described for delivering cooling air during the production of a film tube in the extraction direction. These cooling rings are provided with an annular channel being supplied via a single air supply neck. Cooling rings of similar design are shown and described in JP 59-007 019 A, in JP 58-191 126 A and in JP 58-094 434 A.